Conventional suspension polymerization polyvinyl chloride (herein referred to as PVC) resins have advantageous dry blending properties such as high porosity and low bulk density. However, for processes which require unplasticized extrusion, it is desirable to have a resin with low porosity and high bulk density to assure greater extrusion output rates and less chance of porosity within the product. In the extrusion process, when a high porosity resin is used, air which remains in the resin pores is drawn into the extruder barrel, thus causing porosity in the product.
The instant invention overcomes these problems by producing a product which has low porosity and a high bulk density. This low porosity/high bulk density product is produced by suspension polymerizing vinyl chloride under low shear conditions in the presence of hydroxyethyl cellulose (suspending agent) and an internal lubricant-type compound such as long-chain aliphatic esters in the presence of a minimum amount of oxygen.
The use of internal lubricant-type compounds is not necessary for the production of a high bulk density resin. They are necessary to assure better processing properties of such resins owing to the fact that one of the problems associated with the production of high bulk density PVC is that such resin particles are spherical and glassy. Dry blending these spherical glassy particles with lubricants, processing aids, stabilizers, etc., is difficult, if not impossible, since absorption of these components by such particles is insufficient to form a homogeneous dry blend mixture. The resin begins to thermally decompose at the processing temperature before the additives are uniformly distributed therein. Therefore, the processability as well as the thermal stability of these high bulk density resins are improved by incorporating an internal lubricant-type compound, such as long-chain aliphatic ester into the vinyl chloride during polymerization. That is, each particle or bead of PVC resin contains a small quantity of lubricant; thus facilitating processing.
Addition of lubricants during polymerization is known in the art. For example, U.S. Pat. No. 3,054,786 (Burkholder) teaches the addition of lubricants such as calcium stearate, glyceryl stearate and monoricinoleate during the polymerization process, but only after the desired suspension is effected and prior to discontinuation of polymerization. Burkholder incorporates such lubricants to obtain a high degree of lubricity as well as improving the heat stability of his polymers, but he does not teach the high bulk density resins of the instant invention.